One of the targets in telecommunications is to provide systems where good quality, real-time transmission of video, audio and data services is available. As is generally known, the amount of data needed to transfer moving pictures is high compared to many other types of media, and so far, usage of video in low bit-rate terminals has been negligible. Transmission of data in digital form, anyhow, has provided for increased signal-to-noise ratios and increased information capacity in the transmission channel. In the near future advanced digital mobile telecommunication systems will also be introducing services enhancing the transmission bit-rates, which means that transmission of video even over low bit-rate mobile channels will soon become more feasible.
For optimisation of channel capacity usage, signals are generally compressed before transmission. This is especially important with video transmission, where the amount of data to be transmitted is large. Compressed video, is easily afflicted by transmission errors, mainly because the information content of compressed video is generally coded using variable length codes. When a bit error alters the codeword to another one of different length, the decoder loses synchronisation and decodes consecutive error free blocks incorrectly until the next synchronisation code is received.
To limit the degradations in images caused by transmission errors, error detection and/or error correction methods can be applied, retransmissions can be used, and/or effects from the received corrupted data can be concealed. Normally retransmissions provide a reasonable way to protect data streams from errors, but long round-trip delays associated with low bit-rate transmission and moderate or high error rates make it practically impossible to use retransmission, especially with real-time videophone applications. Error detection and correction methods usually require a large overhead since they add some redundancy to the data. Consequently, for low bit-rate applications, error concealment can be considered as a good way to protect and recover images from transmission errors.
To be able to conceal transmission errors, they have to be detected and localised. The more is known about the type and location of the error, the better the concealment method can be focused to the problem, and accordingly the better image quality will be achieved. It is also important to find methods that can detect especially those errors that are easily detected by the human eye.
Lately, much interest has been attached to error-resilient digital video transmission, but the work has mainly been concentrated on digital TV transmission using MPEG-2. There the problem is solved mainly by adding unique sync codes frequently to the bit stream, using short packets with a cyclic redundancy check (CRC), and discarding all packets where the CRC indicates an error. When the bit-rate of transmission is a few megabytes per second, the proportion of frequently occurring sync codes or CRC fields in the whole data stream is usually acceptable. However, in low bit-rate transmission the situation is quite different, and with bit-rates of 20-30 kbps the optimisation of overheads is extremely important. Furthermore, if the size of the picture is for example 704*576 pixels, one 16*16 pixel macroblock covers about 0.061% of the whole picture, whereas in low bit-rate OCIF (Quarter Common Intermediate Format) 176*144 pixel pictures, one macroblock covers more than 1% of the whole image. Hence, the loss of a macroblock is more detrimental in low bit-rate videophone pictures than in television pictures.
The main interest in low bit-rate video coding standardisation bodies has been to improve error resilience of inter coded frames. Most presented methods suggest changing of the bit-stream syntax and coding algorithms, whereby they can be properly utilised only if they are widely supported by users' videophone terminals. Generally two methods of error detection have been put forward: detection of illegal variable length coding (VLC) code words, and detection of missing end block codes of discrete cosine transform (DCT) matrices. In practice these methods have been found to be insufficient especially for intra coded blocks, since a great many VLC errors remain undetected, and errors in fixed length coded DC components of intra coded blocks are often not detected at all. Furthermore, errors are usually detected far too late, after decoding several corrupted blocks.
The publication of Wai-Man Lam and Amy R. Reibman, “An error Concealment Algorithm for Images Subject to Channel Errors”, in IEEE Transactions on Image processing, Vol. 4, No. 5, pp. 533-542, May 1995 presents some DCT and pixel domain error detection algorithms. These algorithms, however, do not apply adequately to low bit-rates and low resolutions, especially due to the inapplicability of DCT domain algorithms for the different characteristics of quantised DCT matrices.
The publication of Aki Hietala, “Virhesietoinen videodekoodaus”, Master of Science Thesis, Oulu University, Department of Electrical Techniques, 1997, presents and analyses some methods for error detection in video bitstreams. The methods utilise the residual correlation of adjacent pixels (spatial correlation) and by detecting anomalies in block boundaries, search for corrupted blocks. However, the methods are considered rather complex and the achieved effect has not yet been sufficient.
The publication of M. R. Pickering, M. R. Frater, J. F. Arnold, and M. W. Grigg, “An Error Concealment Technique in the Spatial Frequency Domain”, Signal Processing, no.54, Elsevier 1996, pp. 185-189 presents a method for concealing errors that are caused by blocks in the image which are similar in appearance to a single DCT basis function. In the method unusually large DCT coefficients in the 8*8 block of coefficients are detected and reduced to zero. This method works well with specific types of transmission errors, but as a single means of detection has a limited effect.